The server may respond with a 400 Bad Request when we send a review
that adds any invalid reviewers (e.g. an unresolvable account, an
oversized group). We need to not discard the draft review in this
case, and apply special error handling to extract human-friendly
error details from the server response.
Change-Id: I46365c3261f383e9a7a210a622cbb59b1d93d107